Conventionally, a variable geometry turbocharger in which a nozzle for injecting exhaust gas to an exhaust turbine is structured by a pair of exhaust inlet walls facing each other, one of the pair of exhaust inlet walls advancing toward and retreating away from the other to allow adjustment of a gap between the exhaust inlet walls (i.e., opening area of the nozzle), is known. With the variable geometry turbocharger, at a low speed revolution zone of an engine having a small displacement, the gap between the exhaust inlet walls is reduced to increase a flow speed of exhaust gas flowing into the exhaust turbine, thereby increasing the rotary energy of a turbine wheel to enhance supercharging performance of a charging compressor.
A sliding mechanism having a plurality of links is employed for advancing and retreating the exhaust inlet wall, and the slide mechanism is driven by a pneumatic actuator (e.g., Patent Document 1). Here, the pneumatic actuator is typically structured by a cylinder and a piston that slides within the cylinder, and the piston is slid in a first direction by air pressure of a compressed air and in a second direction by a coil spring biasing the piston theretoward. When the piston is slid in the second direction, supply of the air pressure is interrupted.
Also, employment of a hydraulic servo actuator of the four port type instead of the pneumatic actuator is proposed (e.g., Patent Document 2). According to Patent Document 2, though different from aforementioned Patent Document 1 in the mechanism for achieving a variable opening area of the nozzle, a more precise control of an opening degree can be achieved by driving the variable geometry mechanism by the hydraulic servo actuator. The hydraulic servo actuator switches the supply of the pressure oil to the hydraulic chambers on both sides of the servo piston by a proportional solenoid valve. In other words, a position of a spool forming the solenoid valve is switched to switch the supply of hydraulic pressure to the hydraulic chambers.    Patent Document 1: JP-A-11-72008    Patent Document 2: JP-T-2003-527522